Incompatible pointer types initializing 'NSDate

I am setting up some constants, one being an NSDate but receiving this wanring message:

Incompatible pointer types initializing

NSDate *const __strong

with an expression of type

NSString

Simple explanation of code (imp file):

NSDate *const kPAPUserBirthdayKey = @"fbBirthday";

Advanced explanation:
I use a constants file as a singleton holding constant variables for the API i write to. For example the above which is a Date field which will hold the facebook users birthday when connecting to Facebook.